Move ownership of PeerConnectionObserver from Java to C++.

New OwnedPeerConnection takes ownership of the observer. This is done
to allow NativePeerConnectionFactory to return a capsulated object.

Bug: webrtc:8662
Change-Id: Ie876f7b9a1a17ebcfbe51537f712a32ab1a7cbfb
Reviewed-on: https://webrtc-review.googlesource.com/35300
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21610}

sdk/android/api/org/webrtc/PeerConnectionFactory.java

@@ -232,7 +232,7 @@ public class PeerConnectionFactory {
     if (nativePeerConnection == 0) {
       return null;
     }
-    return new PeerConnection(nativePeerConnection, nativeObserver);
+    return new PeerConnection(nativePeerConnection);
   }
 
   /**